This invention relates to a vacuum booster responsive to first and second control valves for operating a master cylinder to provide a brake system with pressurized fluid to effect a brake application. The first control valve responds to a manual input to effect a brake application of all wheel brakes of a vehicle while the second control valve responds to an electrical signal from a controller to selectively effect a brake application of only the drive wheels of the vehicle. When the second control valve receives an actuation signal, pressurized fluid is supplied to an expandable member within a movable wall within the brake booster to interrupt communication of a first fluid to a rear chamber and thereafter initiate communication of a second fluid to the rear chamber to create a pressure differential across the movable wall from which an output force produced. The output force is supplied to the master cylinder to develop pressurized fluid for the brake system. This pressurized fluid is selectively communicated to the drive wheels as a function of the controller to effect a brake application of the drive wheels and thereby synchronize the rotational spin thereof.
Traction control circuits such as disclosed in U.S. Pat. Nos. 4,778,225, 5,096,267 and 5,176,433 have been incorporated into anti-lock brake systems to provide a way of synchronizing the rotational spin of the drive wheels of a vehicle. Unfortunately, during the activation of such traction control systems, the brake pedal, which is directly connected to the movable wall of the vacuum brake booster correspondingly moves or in order to incorporate the traction control structure into the brake system, the vacuum brake booster must be modified through the addition of various internal and external conduits which can significantly increase the cost of a brake system.
Copending U.S. patent application No. 08/068,881 filed May 28, 1993 and now U.S. Pat. No. 5,312,173 discloses a brake booster derived from a standard production brake booster with an add on structure to incorporate traction control through a modification of the input member. The input member is designed to be responsive to either an input force applied through a operator pedal assembly or an input force produced by an in-line traction servomotor which responds to a signal generated in a CPU from sensor signals corresponding to a difference in the rotational spin of the drive wheels to effect a brake application of the wheels in a vehicle. While the in-line traction servomotor would function in an adequate manner, their is an increase number of the overall structural components that must be located between the firewall of a vehicle and the brake pedal.